[Bug c++/38392] New: Template friend function injection

[Bug c++/38392] New: Template friend function injection

[H9XLrv5oXVNvHiUI at spambox dot us]

When defining a friend function in a template class it does not get correctly
defined if the class instantiation comes after the function has been already
called. Note: this happens even if you pre-declare the function in its correct
context before the same function is called. 
The following is a code sample that reproduces the bug. Note that this has been
already discussed and concluded that this is valid c++ standard-compliant code
(precisely here:
http://groups.google.com/group/comp.lang.c++/browse_frm/thread/493afd501c807ffe#).

-- code --

void Function();

int main(int argc, char* argv[])
{
        Function(); // This does not work

}

template <typename T>
class Test
{
public:
        friend void Function() { printf("Function()"); getchar(); }

};

template class Test<int>;

-- end code --


-- 
           Summary: Template friend function injection
           Product: gcc
           Version: 4.2.3
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: H9XLrv5oXVNvHiUI at spambox dot us


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38392

[Bug c++/38392] Template friend function injection

[H9XLrv5oXVNvHiUI at spambox dot us]

-- 

H9XLrv5oXVNvHiUI at spambox dot us changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|normal                      |major


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38392

[Bug c++/38392] Template friend function injection

[H9XLrv5oXVNvHiUI at spambox dot us]

------- Comment #1 from H9XLrv5oXVNvHiUI at spambox dot us  2008-12-16 01:03 -------
Any news?


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38392

[Bug c++/38392] Template friend function injection

[pinskia at gcc dot gnu dot org]

-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|major                       |normal


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38392

[Bug c++/38392] Template friend function injection

[pinskia at gcc dot gnu dot org]

------- Comment #2 from pinskia at gcc dot gnu dot org  2008-12-16 01:08 -------
Confirmed, not a regression.  If you swap around main and the template, the
link works correctly.


-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|0                           |1
           Keywords|                            |link-failure
      Known to fail|                            |3.3.3 4.4.0 4.0.2
   Last reconfirmed|0000-00-00 00:00:00         |2008-12-16 01:08:10
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38392

[Bug c++/38392] Template friend function injection

[H9XLrv5oXVNvHiUI at spambox dot us]

------- Comment #3 from H9XLrv5oXVNvHiUI at spambox dot us  2009-01-31 20:28 -------
Sorry for the late reply!
I thought I'd receive an e-mail when one of my reports gets updated, silly me.

Yes I know it works if you switch the order, but that's exactly the point of
the code, to have them in that specific order and there's no reason why the
code as it is shouldn't work being it standard c++ right?


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38392

[Bug c++/38392] Template friend function injection

[bangerth at gmail dot com]

------- Comment #4 from bangerth at gmail dot com  2009-02-01 05:07 -------
Confirmed indeed, with this (linker) error message:

g/x> c++ x.cc
/tmp/ccjPvb3J.o: In function `main':
x.cc:(.text+0x12): undefined reference to `Function()'
collect2: ld returned 1 exit status

This already fails with gcc2.95.

W.


-- 

bangerth at gmail dot com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |bangerth at gmail dot com
      Known to fail|3.3.3 4.4.0 4.0.2           |3.3.3 4.4.0 4.0.2 2.95.3


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38392

[Bug c++/38392] Template friend function injection

[H9XLrv5oXVNvHiUI at spambox dot us]

------- Comment #5 from H9XLrv5oXVNvHiUI at spambox dot us  2009-09-15 07:37 -------
Any update about this?


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38392

[Bug c++/38392] Template friend function injection

[H9XLrv5oXVNvHiUI at spambox dot us]

------- Comment #6 from H9XLrv5oXVNvHiUI at spambox dot us  2009-12-22 21:52 -------
Did anyone try this against 4.5? Considering that at this stage only bugfixes
are accepted in the codebase for the next release I'd really like to see a
possible fix to this in. I tried to compile GCC 4.5 on my machine without much
success.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38392

[Bug c++/38392] Template friend function injection

[paolo dot carlini at oracle dot com]

------- Comment #7 from paolo dot carlini at oracle dot com  2009-12-22 22:15 -------
Doesn't link with 4.5.0. And doesn't link with ICC and SunStudio either, thus,
I'm rather skeptical it should. I also skimmed quickly through the discussion
on comp.lang.c++ and didn't notice any neat statement... maybe should look
better.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38392

[Bug c++/38392] Template friend function injection

[redi at gcc dot gnu dot org]

------- Comment #8 from redi at gcc dot gnu dot org  2009-12-23 11:02 -------
See http://www.open-std.org/jtc1/sc22/wg21/prot/14882fdis/cwg_defects.html#329

I believe a "use" of the function must come after the definition for it to be
instantiated, indeed if I add this after the explicit instantiation it links
OK:

void(*pf)() = &Function;


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38392

[Bug c++/38392] Template friend function injection

[H9XLrv5oXVNvHiUI at spambox dot us]

------- Comment #9 from H9XLrv5oXVNvHiUI at spambox dot us  2009-12-24 14:48 -------
I know if you move the function it links (btw your link asks me for an HTTP
login), but if you follow the discussion in the newsgroup it was concluded that
this (the above) is actually perfectly valid standard C++ code, and anyway I
think at least the error the linker gives could be more descriptive. Also the
fact that other compilers fail with this code it's just because it's fairly
rarely used code. For instance, Open Watcom compiled and linked the code
perfectly fine without issues (and the executable was working).


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38392

[Bug c++/38392] Template friend function injection

[paolo dot carlini at oracle dot com]

------- Comment #10 from paolo dot carlini at oracle dot com  2009-12-24 15:39 -------
Try this one:

  http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#329

Anyway, if you could point us to the specific sentence in the thread saying
that it's legal, it would be useful. And, well, frankly I would not trust
Watcom more than Intel and Sun (and GCC) together ;) But let's CC Jason, maybe
he wants to give his opinion...


-- 

paolo dot carlini at oracle dot com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jason at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38392

[Bug c++/38392] Template friend function injection

[H9XLrv5oXVNvHiUI at spambox dot us]

------- Comment #11 from H9XLrv5oXVNvHiUI at spambox dot us  2009-12-24 17:23 -------
This is possibly the part in which gets confirmed that the code is standard
compliant, although it reports the exact same paragraph you linked (the old
version):
http://groups.google.com/group/comp.lang.c++/tree/browse_frm/thread/493afd501c807ffe/68c709135884bac7?rnum=11&_done=%2Fgroup%2Fcomp.lang.c%2B%2B%2Fbrowse_frm%2Fthread%2F493afd501c807ffe%3F#doc_38f8441247122dcd

I guess that page is some kind of errata corrige? 
Anyway maybe I'm missing something but I see that as a conflict. If I recall
correctly it is stated that the context of a injected friend function must not
be ambiguous, thus meaning you have to declare it first in the correct
namespace. The idea of declaration is that you inform the compiler that the
function exists somewhere so that you can use it without having it defined yet.
If friend function injection is just like defining the function in the parent
namespace of the class and if a template exists as a type (at linking level)
only when you create an instance of that type, it means the moment you
instantiate the template, then the type is defined and consequently the friend
function is injected, thus matching with the declaration done before, thus
matching with the call that used that declaration.
As I pointed in that newsgroup thread, it works fine with normal classes, and
theoretically a template is just a class in the second you create an instance
of it. I don't precisely understand why there should be an exception for
templates. I understand not defining the function if you don't create any
instance of the template, but otherwise it's a bit unclear to me why there
should be this exception. 


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38392

[Bug c++/38392] Template friend function injection

[jason at gcc dot gnu dot org]

-- 

jason at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         AssignedTo|unassigned at gcc dot gnu   |jason at gcc dot gnu dot org
                   |dot org                     |
             Status|NEW                         |ASSIGNED
   Last reconfirmed|2008-12-16 01:08:10         |2009-12-24 21:47:32
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38392

[Bug c++/38392] Template friend function injection

[jason at gcc dot gnu dot org]

------- Comment #12 from jason at gcc dot gnu dot org  2009-12-24 21:48 -------
Created an attachment (id=19387)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=19387&action=view)
patch

Here's a fix.  I'm going to hold off on applying it for now since it isn't a
regression.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38392

[Bug c++/38392] Template friend function injection

[paolo dot carlini at oracle dot com]

------- Comment #13 from paolo dot carlini at oracle dot com  2009-12-24 21:55 -------
Cool. Should the testcase use dg-do link?


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38392

[Bug c++/38392] Template friend function injection

[jason at gcc dot gnu dot org]

------- Comment #14 from jason at gcc dot gnu dot org  2009-12-24 22:14 -------
Ah, good point.  I've updated the patch accordingly in my local pre-4.6 git
branch.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38392

[Bug c++/38392] Template friend function injection

[H9XLrv5oXVNvHiUI at spambox dot us]

------- Comment #15 from H9XLrv5oXVNvHiUI at spambox dot us  2009-12-24 22:15 -------
Hey thank you! I'd like to test the patch if I only I'd be able to compile 4.5
successfully. You have any idea on when could this patch make it to a final
release?


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38392

[Bug c++/38392] Template friend function injection

[paolo dot carlini at oracle dot com]

------- Comment #16 from paolo dot carlini at oracle dot com  2009-12-25 09:53 -------
As Jason confirmed, this is not a regression, thus, post 4.5.0.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38392

[Bug c++/38392] Template friend function injection

[H9XLrv5oXVNvHiUI at spambox dot us]

------- Comment #17 from H9XLrv5oXVNvHiUI at spambox dot us  2009-12-25 10:42 -------
Ok I managed to compile GCC 4.5, applied the patch and compiled the test code
above and everything works fine. Thanks again!
And yes, I imagined it would have been post 4.5 but I meant 'when' from a time
frame point of view, that's why I said "you have any idea" as I know it's hard
to predict a specific time for release.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38392

[Bug c++/38392] Template friend function injection

[paolo dot carlini at oracle dot com]

------- Comment #18 from paolo dot carlini at oracle dot com  2009-12-25 10:48 -------
If I were you, I would not use this kind of C++ at all, for the time being. As
we discussed already, it's *very* weakly supported and your software would not
be portable.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38392

[Bug c++/38392] Template friend function injection

[H9XLrv5oXVNvHiUI at spambox dot us]

------- Comment #19 from H9XLrv5oXVNvHiUI at spambox dot us  2009-12-25 11:02 -------
Yeah I know, but consider that I used this code only in a specific circumstance
and does not constitute a core part of my application (or a foundation of some
higher level mechanism). And I know it's a lot like an exploit but it does
allow me to add a tiny yet very slick feature to my code. 
Also I checked and they fixed this in visual c++ 2010 as well (after my
report), so having the two major compilers supporting this code is good enough
for me.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38392

[Bug c++/38392] Template friend function injection

[paolo dot carlini at oracle dot com]

------- Comment #20 from paolo dot carlini at oracle dot com  2009-12-25 11:06 -------
Then wait, good luck


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38392

[Bug c++/38392] Template friend function injection

[jason at gcc dot gnu dot org]

------- Comment #21 from jason at gcc dot gnu dot org  2010-04-07 15:55 -------
Subject: Bug 38392

Author: jason
Date: Wed Apr  7 15:54:42 2010
New Revision: 158073

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=158073
Log:
        PR c++/38392
        * pt.c (tsubst_friend_function): Instatiate a friend that has already
        been used.

Added:
    trunk/gcc/testsuite/g++.dg/template/friend51.C
Modified:
    trunk/gcc/cp/ChangeLog
    trunk/gcc/cp/pt.c
    trunk/gcc/testsuite/ChangeLog


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38392

[Bug c++/38392] Template friend function injection

[jason at gcc dot gnu dot org]

------- Comment #22 from jason at gcc dot gnu dot org  2010-05-04 04:54 -------
Fixed for 4.6.


-- 

jason at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
         Resolution|                            |FIXED
   Target Milestone|---                         |4.6.0


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38392